Conventionally, in ordinary household equipment such as home appliances, a switching power supply including a switching power supply control semiconductor device that controls (stabilization or the like) an output voltage through a switching operation by a semiconductor (a switching element such as a transistor) has been widely used as a power supply for the purposes of improving power efficiency by reducing power consumption and the like.
However, the switching power supply generates high levels of switching noise while performing switching operations by turning on/off a switching element, and therefore may potentially have adverse effects on other electronic devices such as malfunction and failure.
In fact, given the need to attain a certain degree of consistency particularly among the standards of different countries in regards to such noise, the international committee CISPR (International Special Committee on Radio Interference) has established and published the EMC (Electromagnetic Compatibility) standard for electronic devices in various fields and automobiles as a “Recommendation”.
In order to suppress switching noise generated by such a switching power supply, parts such as a snubber circuit or a noise filter are generally used. However, in many cases, effectively suppressing switching noise is difficult and requires a large number of noise suppression parts. Consequently, power supply costs and area increase, and considerable effort ends up being spent on measures against noise. Particularly, in recent years, there has been an increase in demands for downsizing and cost reduction in power supplies, and a switching power supply capable of meeting noise standards with fewer noise suppression parts is desired.
In order to satisfy demands for reducing switching noise as described above, a conventional switching power unit is disclosed in Japanese Patent Laid-Open No. 2005-295637, a patent laid-open publication of Japan. The disclosed switching power unit varies a control frequency that controls switching operations according to the amplitude of an AC voltage supplied from an AC input power supply by applying a resistance dividing voltage of a DC current obtained by rectifying the AC voltage by a diode bridge to the control terminal of a PWM control IC that controls switching operations of a switching element.
Performing such control causes the control frequency that controls switching operations of a switching element to increase/decrease according to the amplitude of the AC voltage supplied from the AC input power supply. Therefore, a fluctuation occurs in the control frequency which diffuses switching noise while preventing the switching frequency from concentrating at a constant frequency.
Another switching device that reduces switching noise is disclosed in Japanese Patent Laid-Open No. 2002-354798, a patent laid-open publication of Japan. An RCC (ringing choke converter)-type switching power supply disclosed as an example in Japanese Patent Laid-Open No. 2002-354798 includes a switching frequency variable circuit that continuously varies the switching frequency at a specified period within a range from a first switching frequency to a second switching frequency.
The switching frequency variable circuit includes a series circuit constituted by a transistor and a resistor, a reference voltage supply, a comparator, a memory, a control circuit, and a digital/analog converter, and is arranged to vary the frequency within a range from a frequency slightly higher than two thirds a reference switching frequency to a frequency slightly lower than four thirds the reference switching frequency. This configuration reduces switching noise attributable to switching operations of a switching element.
A noise terminal voltage represents a leakage voltage that is outward leakage of the switching frequency of a switching power supply and harmonic components thereof from a commercial AC power supply-side of the switching power supply. Indicators of a noise terminal voltage include the peak value that is a maximum amplitude value of noise, a quasi peak value (Qp value) that depends on an amplitude or an occurrence frequency of noise and which is close to the maximum amplitude value, an average value, and the like. When the switching frequency is constant, these values remain unchanged and are the same.
On the other hand, while the standard value of an average value is set lower than the standard value of a Qp value, if the Qp value and the average value are the same as described above, the Qp value must be lowered to the average value.
The switching power supplies described in Japanese Patent Laid-Open No. 2005-295637 and Japanese Patent Laid-Open No. 2002-354798 cited above reduce the average value of the noise terminal voltage by diffusing switching frequency.
However, since a conventional switching power supply as disclosed in Japanese Patent Laid-Open No. 2005-295637 employs a PWM control method that is a hard switching method, while diffusing the switching frequency reduces the average value of the noise terminal voltage, the Qp value remains high. Therefore, reliable noise suppression parts for reducing the Qp value become necessary.
In addition, since a conventional switching power supply as disclosed in Japanese Patent Laid-Open No. 2002-354798 employs an RCC method that is a soft switching method, a reduction in switching noise can be achieved with fewer noise suppression parts as compared to Japanese Patent Laid-Open No. 2005-295637. However, the switching frequency variable circuit for diffusing frequency requires more parts, resulting in an increase in power supply cost even though the effort spent on measures against switching noise is reduced.
Meanwhile, with a general RCC-type switching power supply, a switching frequency f can be expressed by the following formula.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                            f        =                                            (                              Vi                -                Vds                            )                        2                                8            ⁢                                                  ⁢                          L              P                        ⁢                          V              o                        ⁢                          I              o                                                          (        1        )            where
Vi: input voltage to transformer,
Vds: drain-source voltage of switching element,
Lp: primary side inductance of transformer,
Vo: output voltage, and
Io: output load current.
From Formula (1) presented above, the following is true.                When the input voltage is constant, the lighter the load, i.e., the smaller the output load current Io, the higher the oscillating frequency f of the switching element.        When the output load current Io is constant, the higher the input voltage Vi, the higher the oscillating frequency f.        
In addition, an input ripple voltage Vi(rip) is determined by the capacity of an input electrolytic capacitor and can be expressed by the following formula.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                      Vi                      (            rip            )                          =                                            2                        ×                          Vi                              (                AC                )                                              -                                                    (                                  2                  ×                                      Vi                                          (                      AC                      )                                        2                                                  )                            -                                                2                  ×                                      P                    o                                    ×                                      (                                                                  1                                                  2                          ×                                                      f                            L                                                                                              -                                              t                        C                                                              )                                                                    η                  ×                                      C                    IN                                                                                                          (        2        )            where
Vi(AC): input AC voltage to transformer,
Po: output power,
fL: commercial frequency,
tC: bridge diode conduction time,
η: power supply efficiency, and
CIN: input electrolytic capacitor capacity.
In a specification of a worldwide AC input power supply, CIN is constant. Therefore, in Formula (2), assuming that variables other than the input voltage Vi(AC) are constant, it is found that the higher the input, the smaller the input ripple voltage Vi(rip). For example, if P0=60[W], fL=50 [Hz], tC=2 [ms], η=0.8, and CIN=150 [μF], when Vi(AC)=100 [V], Vi(rip)=31.9 [V] However, when Vi(AC)=240 [V], Vi(rip)=12 [V].
With an RCC-type switching power supply, according to Formula (1), since the oscillating frequency f varies with input voltage Vi, input ripple voltage is high during low input. As a result, the variation in oscillating frequency increases, causing diffusion of oscillating frequency over a wide range. However, input ripple voltage during high input is low and the variation in oscillating frequency is significantly small.
As described above, in the case of the RCC method, oscillating frequency requisitely diffuses due to input ripple voltage and achieves an effect of reducing the noise average value. However, during high input, since the diffusion of oscillating frequency due to input ripple voltage is small, the noise average value tends to deteriorate.
While the input electrolytic capacitor capacity can conceivably be reduced during high input, in order to ensure that the oscillating frequency diffuses, the reduction in the input electrolytic capacitor capacity disadvantageously increases input ripple voltage during low input significantly, which in turn shortens the life cycle of the input electrolytic capacitor.
Furthermore, in the case of DC input instead of AC input, a diffusion of the oscillating frequency due to input ripple voltage as described above does not occur. Therefore, if the output load is constant, the oscillating frequency of the switching element becomes fixed and the noise average value becomes equal to the Qp value, thereby necessitating adequate measures against noise.